Known direct-current drives can be operated by power converters which are either supplied by an AC voltage source or from a DC voltage source. A variable direct current is generated by a power converter which is usually operated via phase gating control, by which variable direct current the direct-current drive can be operated according to the respective specifications. One example for a power converter is a B6 bridge populated with thyristors. Industrial direct-current drives have, for example, a nominal voltage in the range of up to 1500 V at an electrical power of up to 20 MVA, wherein, depending on the application, characteristic variables which deviate significantly therefrom can also occur.
In the event of a safety-related disconnection of a direct-current drive, care should be taken that the driving moment thereof becomes zero as quickly as possible in order to promptly exclude the risk of possible danger. For this purpose, it is sufficient that either the armature current or the field current has decayed to zero. Such a functionality is usually referred to as a safe torque off (STO) function. A reduction in the armature current, which is fed in by the power converter, is of particular interest in this connection.
Because a grid-supplied power converter cannot be immediately de-energized, at least in the case of a high output voltage during regenerative operation (during braking of the direct-current machine), a corresponding time period is called for, in which the current can be reduced. A corresponding time period which is the maximum permissible in terms of safety from giving a signal to commence the STO function until reducing the torque of the direct-current drive to a value of zero is, for example, in the range from 500 ms to 1000 ms, depending on the respective boundary conditions.
According to known implementations, when an STO signal is given, the output voltage of the armature power supply is adjusted such that the current becomes smaller as quickly as possible and the armature current can be reduced within the stated exemplary time interval. After said interval has elapsed, the power converter is then decoupled from the direct-current drive and/or the power converter is decoupled from its infeed by a mechanical interruption device, for example a contactor. By way of example, patent document EP 2220757 B1 likewise relates to said problem.
Here, it proves to be disadvantageous that—in particular in the relatively high power range, mentioned at the outset, of the direct-current drives—the maximum permissible number of switching cycles of mechanical interruption devices before said devices should be exchanged or at least maintained is relatively small. Depending on the frequency of initiation of a safe torque off (STO) function during operation, it is possible for excessive wear on the interruption devices to occur.